Apparatus and Method for Producing Electrospun Fibers

ABSTRACT

An apparatus for making electrspun fibers comprises a collector that may be submerged in a coagulation bath. The collector may be automatically movable between a first position and a second position, wherein at least a portion of the collected fibers are submerged in a coagulation bath in the first position and spaced apart from the coagulation bath in the second position. The collector may be a rotating collector. A process for making electrospun fibers comprises electrospinning a dispersion and collecting a plurality of electrospun fibers, followed by submerging the collected fibers in a coagulation bath.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application claims the benefit of U.S. Provisional SerialApp. No. 60/747,340, which is incorporated by reference herein in itsentirety. The instant application further claims the benefit of U.S.patent application Ser. No. 10/965,813, which, in turn, claims thebenefit of U.S. Provisional Serial App. No. 60/583,358, both of whichare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to fibers and more particularly to methodsfor making cellulose nanofibers.

BACKGROUND

Various naturally occurring polymers are of particular interest due totheir abundant availability and biodegradability. Cellulose, forexample, is one of the most abundant naturally occurring polymers onearth, and finds use in countless applications. Yet, cellulose hasproven difficult to transform into fibers because of its relatively highdegree of intermolecular and intermolecular hydrogen bonding, whichrenders it substantially insoluble in common solvents. The effectivemanufacture of cellulose fibers, therefore, is an area of interest.

SUMMARY OF INVENTION

The present invention relates to improved fibers comprising organicpolymers and methods for making the same.

In one aspect, a process for making organic fibers compriseselectrospinning a dispersion comprising an organic polymer. In oneembodiment, the process comprises providing a starting materialcomprising an organic polymer, dissolving the starting material in apolar solvent to create a dispersion, electrospinning the dispersion byproviding an electric charge on droplets of the dispersion to produce acharged jet of polymer and provide a plurality of electrically inducedbending instabilities and/or whipping motions, collecting a plurality ofthe charged jets as fibers and submerging the plurality of fibers into acoagulation bath as they are collected. In another embodiment, theprocess comprises providing a starting material comprising cellulose,dissolving the starting material in a polar solvent unreactive withcellulose to create a dispersion, electrospinning the dispersion byproviding an electric charge on droplets of the dispersion to produce acharged jet of polymer and provide a plurality of electrically inducedbending instabilities and/or whipping motions, collecting a plurality ofsubstantially amorphous non-derivatized cellulose nanofibers on arotating collector and submerging the rotating collector comprising theplurality of substantially amorphous non-derivatized fibers into acoagulation bath on a periodic basis. The collector may, for example, besubmerged in regular time intervals between about 1 and about 3.

The above-identified processes may incorporate various additionalfeatures and steps. The solvent used in the dissolving step may, forexample, be sufficiently volatile to substantially dissolve during theelectrospinning step and can comprise lithium chloride and N,N-dimethylacetamide. The electrospinning step may be carried out in anelectric field between about 1.0 kV/cm to about 4.0 kV/cm at a flow ratebetween about 0.03 ml/min. and about 0.05 ml/min. Further, co-axialelectrospinning may be employed. After the collecting step, therecovered fibers may be exposed to at least one of water and alcohol forremoving residual solvent without dissolving the fibers or heating attemperatures between about 70° C. and 110° C. Typically, the collectedfibers exhibit substantially uniform diameters. The fibers may comprisesubstantially uniform diameters and a degree of crystallinity betweenabout 1% and about 40%. The fibers may also comprise a substantiallyamorphous form of cellulose exhibiting an X-ray diffraction patterncomprising at least two 2θ values selected from about 7.0 degrees andabout 17.8 degrees. The coagulation bath may comprise at least one ofwater and alcohol for removing residual solvent without dissolving thefibers.

In another aspect, an apparatus for making fibers comprises a chamber,pump, syringe, voltage supplier and syringe. In one embodiment, theapparatus comprises a chamber for receiving a dispersion, a pumpupstream of the chamber for dispensing the dispersion, a syringe forreceiving the dispersion from the pump and comprising an opening forproviding droplets of the dispersion, a voltage supplier in electricalcommunication with the syringe and a collector. The voltage supplierprovides an electric charge in the droplets emanating from the openingto produce a plurality of charged jets and provide a plurality ofelectrically induced bending instabilities and/or whipping motions. Thecollector receives and collects the charged jets as non-woven fibers andautomatically moves between a first position and a second position,wherein at least a portion of the collected fibers are submerged in acoagulation bath in the first position and spaced apart from thecoagulation bath in the second position. The collector may be a rotatingcollector.

The apparatus may further comprise a heating chamber constructed of anelectrically and thermally insulating material, shielded from thevoltage supplier. Additionally or alternatively, a heating gun forheating the collector may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention are illustrated by theaccompanying figures. It should be understood that the figures are notnecessarily to scale and that details not necessary for an understandingof the invention or that render other details difficult to perceive maybe omitted. It should be understood, of course, that the invention isnot necessarily limited to the particular embodiments illustratedherein.

FIG. 1A is a scanning electron microscope (“SEM”) image of cellulosenanofibers made in accordance with one embodiment of the presentinvention;

FIG. 1B is an SEM image of FIG. 1A;

FIG. 2B is an SEM image of nanofibers on filter media for dustcollection made in accordance with a second embodiment of the presentinvention;

FIG. 2A is an SEM image of FIG. 2A;

FIG. 3A is a perspective view of one embodiment of an electrospinningapparatus used to prepare the nanofibers of the present invention;

FIG. 3B is a perspective view of an alternate embodiment of theelectrospinning apparatus of FIG. 3A;

FIG. 4 is a graph illustrating the volatility of DMAc, with massmeasurements plotted as a function of time; and

FIG. 5 is an X-ray diffraction pattern for the cellulose startingmaterial, and cellulose nanofibers made in accordance with the presentinvention before and after coagulation.

DETAILED DESCRIPTION

The nanofibers of the present invention may be made from organicpolymeric starting materials. These organic polymeric starting materialsare typically mixed with a solvent comprising a relatively highvolatility (i.e., the readiness with which a material vaporizes), andthen subjected to an electrospinning process. Ideally, the solvent issufficiently volatile to substantially dissolve during theelectrospinning process.

Nanofibers made in accordance with the present invention comprise anorganic polymer, and typically exhibit fiber diameters between about50.0 nm and 1.0 micron, more particularly between about 100.0 nm and300.0 nm and still more particularly between about 100.0 nm and 200.0mm. The nanofibers may be relatively amorphous, comprising a degree ofcrystallinity less than about 60% and more particularly between about1.0% and 50.0% and still more particularly between about 1.0% and 40.0%.The degree of crystallinity is determined through X-ray diffractionpatterns and may be modified by adjustments to the electrospinningprocess, including spinning distance, flow rate and spinningtemperature. The specific surface area of the nanofibers may be betweenabout 1.0 m²/g and 50.0 m²/g and more particularly between about 5.0m²/g and 10.0 m²/g, as measured by the BET surface area testingmethodology. The degree of polymerization of the nanofibers may bebetween about 50 and about 2000 and more particularly about 200 to about1150.

When the nanofibers are randomly dispersed, with at least some of thenanofibers in physical contact with one another, a nanofiber mat isformed. The nanofiber mat can comprise a plurality of pores orinterstices between individual fibers. The pores typically comprisediameters between about 0.5 microns and 3.0 microns.

The nanofibers may be composed of various materials, such ascellulose-based polymers. In one embodiment the nanofibers comprisenon-derivatized substantially amorphous cellulose, which comprises therepeat unit:

in random orientations and positions. SEM images of a plurality ofcellulose nanofibers made in accordance with certain embodiments of thepresent invention are shown at FIGS. 1A-2B.

The nanofibers and nanofiber mat of the present invention may be madethrough a step-wise process. Typically, a starting material ispre-treated and subjected to electrospinning, with heating and anoptional coagulation step.

Various starting materials may be employed, including withoutlimitation, organic polymers such as cellulose. The starting materialsare often substantially insoluble in common solvents like water. Themolecular weight of the starting material may be between about 10,000and 325,000 g/mol and more particularly between about 30,000 and 200,000g/mol. The degree of polymerization of the starting material may bebetween about 50 and about 2,000 and more particularly about 200 toabout 1,150. Suitable examples of cellulose starting materials includecotton linter paper, cotton batting, recycled cellulose and purifiedbast fibers.

The starting material is typically ground into fine particles and soakedin water to break or weaken hydrogen bonding. The starting material maybe ground to particle sizes between about 5.0 mesh and 50.0 mesh andmore particularly between about 15.0 mesh and 25.0 mesh. The finelydivided starting material is placed in water and soaked at roomtemperature for a period of between about 6.0 hours to about 15.0 hoursand more particularly between about 8.0 hours and 12.0 hours. The watermay be high performance liquid chromatography water available fromMallinckrodt of Phillipsburg, NI. The starting material is thereafterdried under vacuum at between about 55° C. and about 65° C.

After the starting material is dried, it may be dissolved in a solvent,comprising a relatively high volatility. Preferably, the solvent doesnot chemically react with the starting material and is sufficientlyvolatile to substantially dissolve during the electrospinning process.The term “solvent” as used herein means any compound or substance ormixture of liquid compounds or substances used to dissolve part or allof the starting material.

The solvent may, for example, be a polar solvent used to help weaken orbreak hydrogen bonding within the organic starting material. Theloosening or breakage of hydrogen bonds enhances the solubility of thestarting material within the solvent. Polar solvents are beneficialduring the electrospinning step because of their relatively highconductivities. In addition, depending on the selection of the solvent,dissolution may proceed without side reactions, leading to anon-derivatized end product. In one embodiment, the solvent comprisesDMAc, to which lithium chloride may be added. This solution has beenshown to dissolve cellulose from different sources over a large range ofconcentrations without side reactions. The presence of lithium chloridebridges electrostatic interactions between cellulose and DMAc. In otherembodiments, solutions of NMMO/H₂0 may be employed. Dissolutiontypically proceeds for about 2.0 hours under constant stirring, withmild heating between about 50° C. to about 60° C. The finalconcentration of starting material in the solution may be between about1.0% by weight to about 10% by weight and more particularly betweenabout 3.0% by weight to about 6% by weight. The final solution mayexhibit a zero shear viscosity between about 2,500 Pa*s and 3,500 Pa*s.After dissolution of the starting material in the solvent, thedispersion is subjected to electrospinning.

Electrospinning is a fiber formation process that relies on electrical,rather than mechanical forces to form thin fibers (sub-micron fibers forexample). With electrospinning, an electric field is used to draw asolution from the tip of a capillary to a grounded collector. Theelectric field causes a pendant droplet of the solution at the capillarytip to deform into a conical shape. When the electrical force at thesurface of the tip overcomes the surface tension of the solution, acharged jet is ejected and undergoes a series of electrically inducedbending instabilities, whereby repulsion of adjacent charged segmentsgenerates vigorous whipping motions, which elongate the charged segmentsinto fibers for passage onto a collector. The solvent begins toevaporate after jet formation, causing the deposit of thin fibers on thecollector. To the extent residual solvent remains, the collected fibersmay be heated to about 150° C. for removal thereof.

Referring now to FIGS. 3A and 3B, two embodiments of an electrospinningapparatus 100 for use with the present invention are illustrated.Apparatus 100 comprises syringe 102, tip 104, high voltage supplier 106positioned at or near tip 104, micropump 108, positioner 109, heatingunit 110, rotating collector 112, coagulant bath 114 and motor 116 fordriving rotation of collector.

As shown in FIGS. 3A and 3B, syringe 102 is positioned horizontally onmicropump 108 and typically comprises an inner diameter between about0.10 millimeters to about 0.60 millimeters. The diameter of collectednanofibers may be decreased by decreasing the inner diameter of syringe102. Micropump 108 may be a PHD 2000 Infusion syringe pump, availablefrom Harvard Apparatus, Inc. of Holliston, Mass. Positioner 109 may beused to control the height of micropump 108.

Voltage supplier 106 may be set between about 1 OkV to about 30 kV andmore particularly between about 15 kV and about 25 kV. Voltage supplier106 provides an electric field between about 1.0 kV/cm to about 4.0kV/cm.

Collector 112 may be mesh or a plate and constructed of a conductivematerial, such as aluminum, stainless steel or a surface oxidizedsilicon. Collector 112 may also comprise a flat sheet of non-wovencellulose, mixed with about 10% to about 20% polyester fibers.

Collector 112 is grounded to create an electric field difference betweentip 104 and collector 112, allowing material to move from the highelectric field at tip 104, to grounded collector 112. The distancebetween tip 104 and collector 112 may be between about 5.0 cm and 15.0cm and more particularly between about 7.0 cm and 12.0 cm. A steppermotor 116 may be connected to collector 112 to provide continuousrotation into coagulant bath 114 at predetermined intervals. Theintervals may be between about 1.0 to about 10.0 revolutions per minuteand more particularly between about 3.0 to about 5.0 revolutions perminute.

Once apparatus 100 is assembled, a solution comprising the startingmaterial and solvent is placed into syringe 102, and pumped therethroughat a relatively constant flow rate of about 0.03 milliliters per minuteto about 0.05 milliliters per minute. As pumping continues, a chargedjet is ejected and elongates as it moves towards collector 112. Aplurality of randomly oriented substantially dry non-woven fibers arecollected on collector 112. The collected fibers typically exhibituniform diameters (i.e., substantially all the collected fibers exhibitthe same or similar fiber diameters)

In an alternate embodiment, co-axial electrospinning may be employed.Co-axial electrospinning employs a dual syringe which comprises aninternal tube positioned within an external tube. Under thisconstruction, an internal jet within an external jet is ejected from thesyringe; the internal jet may comprise organic substances, such asmineral oil, while the external jet comprises the aforementionedsolution. When mineral oil is used, hollow nanofibers or nanotubesemerge. The term nanofiber, as used herein, is intended to covernanotubes.

The presence of residual solvent in the collected nanofibers can lead tounwanted clumps or film-like structures on nanofiber surfaces. There arevarious ways to decrease residual solvent.

To enhance evaporation of solvent during processing, a heating step maybe employed. The heating step causes vaporization of the relativelyvolatile solvent, while leaving the starting material substantiallyintact. As shown in FIG. 4, for example, DMAc evaporates rapidly attemperatures beyond 50° C.

Heating may be carried out through heating unit 110 or electric heatingguns. Heating unit 110, the features of which are described inco-pending co-owned U.S. patent application Ser. No. 10/965,813, heatsthe solution as it travels through syringe. Heating unit 110, typicallycomprises an electrically and thermally insulating material and may beshielded from high voltage supplier 106 to prevent induced voltage inthe heating source. A Faraday cage or screen, comprising an enclosuremade of metal mesh, may serve as the shield. Heating unit 110 istypically used for cellulose mixed with solutions of NMMO/water attemperatures ranging between about 70.0° C. and about 110.0° C. forabout one hour. Alternatively or additionally, collector 112 may beheated to between about 90.0° C. and about 120.0° C. and moreparticularly between about 100.0° C. to about 110.0° C. Commerciallyavailable electric heating guns may be utilized for this purpose. Suchelectric heating guns are often used in connection with cellulosesolutions comprising DMAc and LiCl. By applying heat to collector 112rather than the system as a whole, the viscoelastic properties of thesolution up to formation of jet are conserved. Heating in this manneralso does not significantly degrade the starting material because of therelatively rapid evaporation of the more volatile solvent at elevatedtemperatures.

Coagulation bath 114 may also be used to remove solvent from theelectrospun fibers. In the DMAc/LiCl system, the presence of hygroscopicsalt causes localized moisture absorption, which leads to the formationof water droplets on the intersection of the collected fibers andunwanted fiber swelling. Coagulation bath 114 helps removes residualLiCl by dissolving it. X-ray diffraction patterns, shown at FIG. 5,confirms the removal of salt from the collected fibers, as thecharacteristic peak of salt, at 2θ=30.09, is only observed in theuntreated fibers and essentially disappears after coagulation. With theNMMO/water system, residual NMMO is exchanged with the contents ofcoagulation bath 114.

Coagulation bath 114 may comprise any substance or solution that removesresidual solvent but does not dissolve the starting material within thecollected fibers. Suitable examples include analytical grade water oralcohol. Exposure to coagulation bath may occur about 1.0 to 3.0 secondsafter the substantially dry fibers have been electrospun onto collector112. This 1.0 to 3.0 second interval is automatically maintained bycontrolling the rotation speed of rotating collector to between about1.0 to about 10.0 revolutions per minute and more particularly tobetween about 3.0 to about 5.0 revolutions per minute. Collected fibersmay be exposed to coagulation bath 114 for about 30.0 to 40.0 minutes.

It bears noting that in the case of cellulose based fibers, collector112 may comprise cellulose filter media. Under these circumstances, thesurrounding cellulose filter media is adapted to distribute moistureabsorption uniformly throughout the electrospun fibers, therebypreventing formation of large droplets of water that lead to swelling ofthe fibers.

Practice of the above-described methods yields nanofiber mats comprisinga plurality of nanofibers typically constructed of a non-derivatizedorganic polymer. The term non-derivatized, as used herein, means thatalthough residual solvent may be physically entrapped within the matrix,the atoms or elements of the starting material have not been substitutedor replaced with the atoms or elements of another material. For example,a non-derivatized cellulose nanofiber comprises the structure of therepeat unit for cellulose shown above, without substitution orreplacement of atoms.

The present invention may be used in a variety of different ways,including, without limitation, in sensing and filtering applications.For instance, nanofibers made in accordance with the present inventionmay be used for filtration of sub-micron dust particles. The collectionefficiency of the nanofibers is relatively high, with collection ofabout 30% to about 50% of dust passed through a filter comprising thenanofibers of the present invention is typical. The nanofibers are alsouseful in medical applications. The nanofibers may be used as ahemostatic wound dress, to mimic the formation of fibrin to aid in bloodclotting at wound surfaces. In addition, the nanofibers may be used as abarrier after surgery. Since the nanofibers with small fiber dimensioncan comprise amorphous cellulose, they typically degrade faster thanconventional, thick cellulose membranes. With fibers of the presentinvention, the degradation process within the body occurs in about threeto five days. Additionally, the need to dispose of conventionalcrystalline cellulose in landfills and the like is substantiallydecreased.

While certain embodiments of the present invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the spirit andscope of the invention. The matter set forth in the foregoingdescription and accompanying drawings is offered by way of illustrationonly and not as a limitation. The actual scope of the invention isintended to be defined in the following claims.

1. A process for making fibers comprising: providing a starting materialcomprising an organic polymer; dissolving the starting material in apolar solvent to create a dispersion; electrospinning the dispersion byproviding an electric charge on droplets of the dispersion to produce aj et of polymer and provide a plurality of electrically induced bendinginstabilities and/or whipping motions; collecting a plurality of thejets as fibers; and submerging the plurality of fibers into acoagulation bath as they are collected.
 2. The process of claim 1,wherein the organic polymer used in the providing step is cellulose. 3.The process of claim 1, wherein the polar solvent used in the dissolvingstep comprises lithium chloride and N,N-dimethylacetamide.
 4. Theprocess of claim 1, wherein the polar solvent is sufficiently volatileto substantially dissolve during the electrospinning step.
 5. Theprocess of claim 1, wherein the electrospinning step is carried out inan electric field between about 1.0 kV/cm to about 4.0 kV/cm at a flowrate between about 0.03 ml/min. and about 0.05 ml/min.
 6. The process ofclaim 1, wherein the fibers are nanofibers comprising diameters betweenabout 100 nm and 300 nm.
 7. The process of claim 1, wherein the fiberscomprise a degree of crystallinity between about 1% and about 40%. 8.The process of claim 1, wherein the fibers comprise a substantiallyamorphous form of cellulose exhibiting an X-ray diffraction patterncomprising at least two 2θ values selected from about 7.0 degrees andabout 17.8 degrees.
 9. The process of claim 1, wherein the fiberscomprise a repeat unit conforming to the general formula:

in random positions and orientations.
 10. The process of claim 1,wherein the coagulation bath comprises at least one of water and alcoholfor removing residual solvent without dissolving the fibers.
 11. Theprocess of claim 1, wherein the fibers are collected on a rotatingcollector and the rotating collector is submerged into a coagulationbath on a periodic basis.
 12. The process of claim 1, further comprisingheating the rotating collector to temperatures ranging between about 70°C. and 110° C.
 13. The process of claim 12, further comprising shieldinga voltage collector for supplying an electric charge from heating. 14.The process of claim 1, wherein the collector comprises a cellulosefilter media.
 15. A process for making cellulose nanofibers comprising:providing a starting material comprising cellulose; dissolving thestarting material in a polar solvent unreactive with cellulose to createa dispersion; electrospinning the dispersion by providing an electriccharge on droplets of the dispersion to produce a charged jet of polymerand provide a plurality of electrically induced bending instabilitiesand/or whipping motions; collecting a plurality of the charged jets assubstantially amorphous non-derivatized cellulose nanofibers on arotating collector; and. submerging the rotating collector comprisingthe plurality of substantially amorphous non-derivatized fibers into acoagulation bath on a periodic basis.
 16. The process of claim 15,wherein the fibers are submerged in the coagulation bath at regularintervals between about 1 and about 3 seconds.
 17. An apparatus formaking fibers through electrospinning comprising: a chamber forreceiving a dispersion; a pump upstream of the chamber for dispensingthe dispersion; a syringe for receiving the dispersion from the pump,the syringe comprising an opening sized to providing droplets of thedispersion; a voltage supplier in electrical communication with thesyringe to provide an electric charge in the droplets emanating from theopening to produce a plurality of charged jets and provide a pluralityof electrically induced bending instabilities and/or whipping motions; acollector for receiving and collecting the charged jets as non-wovenfibers, the collector automatically movable between a first position anda second position, wherein at least a portion of the collected fibersare submerged in a coagulation bath in the first position and spacedapart from the coagulation bath in the second position.
 18. Theapparatus of claim 17, wherein the collector comprises a rotatingcollector.
 19. The apparatus of claim 18, wherein the rotating collectoris submerged in the coagulation bath on a periodic basis.
 20. Theapparatus of claim 17, further comprising a heating chamber constructedof an electrically and thermally insulating material, the heatingchamber positioned to heat the dispersion as it travels through thesyringe.
 21. The apparatus of claim 20, wherein the heating chamber isshielded from the voltage supplier.
 22. The apparatus of claim 17,further comprising a heating gun for heating the collector.
 23. Theapparatus of claim 17, further comprising a motor for driving thecollector.